This invention relates to a method and apparatus for detecting the presence of predetermined compounds in a sample, particularly to liquid and gas chromatographic detection techniques for application to chromatographic analysis of samples containing nitrogen, sulphur, carbon and halogen compounds.
Systems utilizing chromatographic techniques for separation of components in samples are well known in the art. All such systems rely on differential migration processes where the components of a sample in a moving phase are selectively retained by a stationary phase. The moving phase may be a gas, as in gas chromatographic systems, or a liquid, as in liquid chromatographic systems. The stationary phase in either of such systems may be either liquid or a solid.
The early Nobel prize winning work of A. J. P. Martin and R. L. M. Synge (Biochem. J. 35, 91, 1358 (1941) set forth the basic liquid chromatographic techniques used in systems today. However, practical applications for these techniques have been severely limited by detector development. As a result, liquid chromatographic analysis has been generally a lengthy procedure, often taking hours and even days.
Recently, detectors characterized by relatively high sensitivity, low noise, and wide linear response range, have become increasingly available. Such detectors include UV absorption, refractive index, micro-adsorption and flame ionization detectors, and the more limited range electrical conductivity and fluorescence detectors. Furthermore, high pressure fluid pumps (in excess of 5000 psi) have also become available, permitting the use of long, narrow bore (e.g. 1 mm) columns having small diameter packing particles. This combined development of high pressure pumps and high performance detectors for use with high performance narrow bore columns, has reduced the time required for liquid chromatographic analysis from hours to minutes in many cases. However, even using such elements, the known liquid chromatographic detection systems only provide analysis of liquid samples with sensitivity as high as one part in 10.sup.6 in particularly favorable cases, such as where the compound has a strong UV absorption band for UV detection at the corresponding wavelength.
Gas chromatographic detection systems have been developed primarily since the paper by A. J. P. Martin and A. T. James (Analyst 77, 915 (1952)). One disadvantage of such systems, as compared to liquid detection systems, is the occurance of breakdown of a liquid sample at the vaporization temperatures applied at the input to the column and at the high temperatures applied at the column itself (these latter temperatures being maintained to decrease the retention time of the column). In spite of this disadvantage, gas chromatographic systems have received far more attention than the liquid detection systems because of the availability of higher sensitivity and faster response detectors. Specifically, known gas detectors include the flame ionization detector (FID) which is highly sensitive to any compound containing carbon, the electron capture detector (EC) which is highly sensitive to halogen compounds, and the thermal conductivity detector (TC) which is highly sensitive to all compounds (a general or "universal" detector). However, these detectors also have significant sensitivities to other compounds which may mask responses from desired compounds, thereby rendering such detectors unsuitable for use in the detection of those desired compounds. For example, in the case of halogen detection using an electron capture detector, both the alcohol and halogen compound content of the sample produce interfering responses. For the specific detection of nitrogen compounds, two commercial detectors are available: the Coulson detector and an adaptation of the FID. Both have nitrogen sensitivities substantially less than one part in 10.sup.9 and both are extraordinarily difficult to operate.